Laser acceleration of electrons to multi-Mev energies has been demonstrated theoretically (1) and experimentally (2,3) within the last twenty years. Most recently, the first use of laser accelerated electrons to produce multi-MeV ion beams has been experimentally observed (4) by a number of laboratories in the U.S. and the U.K. The Department of Energy Laboratories, under the national Laser Fusion Program, utilizes a technique known as inertial confinement. This requires an enormous laser device, with a large number multiple beams to produce a relatively uniform implosion reaction at the surface of a fusion pellet target to produce the required compression to ignite a fusion reaction at the pellet fuel core. The only other fusion alternative has been the long running magnetic confinement fusion program, that has spanned over fifty years. Both programs require huge expenditures and large facilities to produce any fusion reactions.
The present invention offers the potential of creating a significant level of fusion reactions (of the order of 10e+12 reactions per pulse) within a laboratory setting typically found in many university laboratories, which would require minimal space, and expenditures in the hundreds of thousands of dollars as opposed to the hundreds of millions of dollars spent on the national facilities. It is also possible to produce fusion reactions using a single ion beam by using a single foil composed of one of the fusion reactants, backed by a second solid or gaseous medium composed of the second fusion atom reactant species. Colliding beams produce fusion reactions at lower beam energies than stationary target configurations.
The major component of this invention is a high powered, short pulse, chirped pulse amplification laser system. Typical parameters required to create the electron acceleration levels necessary to pull ions to levels required in fusion reactions are; pulse energies of the order of 0.5 Joule, pulse widths of several hundred femtoseconds or less, and focal spot diameters of the order of a few microns. This combination should produce the requisite energy densities of greater than 10e+18 watts/cm2 necessary to create the electric field strengths required to ionize gas or foil targets and then accelerate the electrons ejected from the atoms to multi-Mev energies. At incident energy densities of this level, the laser beam can create an ionization channel that might extend from several hundred microns to the order of a centimeter, due to self-focusing and/or channeling mechanisms.
The novel configuration that allows fusion energy production, not possible in a typical black-body plasma that could be created by this amount of energy density, is the colliding beam geometry that provides the maximum center of mass energy for fusion reactions. Uniform plasma configurations have to deal with both ion-ion and ion-electron collisions, and an optimal collision geometry does not always exist in these black-body plasmas. The colliding beams can be produced by splitting the laser accelerator pulse into two beams, and using turning mirrors and separate focusing optics to create counter-propagating laser pulses. Alignment, although required to be accurate to the order of a micron, should be feasible with current technology.
Recent experiments have used solid foil targets to create the ion beams, which exit from the rear of the foil. This configuration probably allows the ions to be accelerated without the return of electrons to slow down the ions after the end of the laser pulse, since the electrons"" fields are likely xe2x80x9cshorted outxe2x80x9d by the back plane of the target foils. An alternative configuration may be possible using pulsed gas targets, with additional foils to create similar electrostatic environments. Gas targets can have channels as long as a centimeter, whereas foil targets are typically of the order of tens of microns. In order to stabilize long channels in gas targets, it may be possible to utilize a capillary waveguide to create a more uniform ionized channel region. These capillary waveguides (of the order of tens of microns or more in diameter) can also be used to alter the effective index of refraction of the channel medium, and hence the laser field phase/group velocity, which may be useful for x-ray production applications where timing adjustments to synchronize spontaneous emission is necessary.
The number of ions accelerated will depend on the relative densities of the targets; ratios of solids to gas densities are typically of the order of 10e+4 and thus the relative thicknesses of gas and solid targets should be of the order of this ratio. Pulsed magnetic fields of the order of a few megagauss (which could be obtained from a lower powered laser pulse, polarized and directed normal to the ion direction) could contain the ion beams over reasonable foil separation distances, but having the two foils in close proximity (of the order of 30 microns) would be much simpler and more energy efficient, given the 30 degree beam divergences reported for the ion beams in the literature.